This invention relates to a noise reduction system and, more particularly, to such a system which is useful in reducing the noise in the higher frequency portions of a signal such as a composite video signal, without imparting significant distortion into the noise-reduced version of the input signal.
The power distribution characteristic of the frequency spectrum in a composite video signal is such that the lower frequency components generally exhibit higher signal levels than the higher frequency components. Consequently, the signal-to-noise (S/N) ratio is much higher for the lower frequency components than for the higher frequency components of such a signal. This means that if noise is uniformly distributed over the frequency spectrum, such noise usually will not deleteriously influence the lower frequency components of the video signal because of the higher S/N ratio. However, since the S/N ratio is not as good in the higher frequency portion of the video signal spectrum, it is desirable to suppress such noise.
In one noise reduction proposal, the low frequency and high frequency components of the video signal are separated from each other, typically by low-pass and high-pass filters. Then, the noise which accompanies the high frequency components, such as noise produced by a video tape recorder (VTR) used to record and play back video signals, is suppressed. Such noise suppression is carried out on the basis of the relatively poor S/N ratio exhibited by the higher frequency portion of the video signal. That is, it is assumed that if the signal level of the higher frequency components is less than a threshold level, then, because of this poor S/N ratio, such low-level signals are noise. Accordingly, the higher frequency signal level that does not exceed the threshold level is suppressed. Following this signal suppression stage, the separated lower frequency and higher frequency components are recombined so as to reconstruct essentially the input video signal.
Unfortunately, if filters are relied upon for separating the higher and lower frequency components of the input video signal, the fact that such filters generally impart unequal phase shifts to the signals applied thereto results in phase distortion when the low frequency components are recombined with the high frequency components. Also, the amplitude-frequency characteristics of these filters generally are not equal. Hence, because of the different phase-shifts and the different amplitude characteristics in the recombined lower frequency and higher frequency components attributed to these filters, the resultant video signal exhibits significant distortion which is detectable in the reproduced video signal.